The instant invention relates to optoelectronic communication systems. More particularly, the present invention relates to a fiber optic module including an optical subassembly (OSA) coupled to an integrated circuit or other electronic device, by means of a printed circuit board or other mounting device.
In today's rapidly advancing optoelectronic industry, vertical cavity surface emitting lasers (VCSELs) have become preferred as the optical source for providing data signals. VCSELs are favored because of the ease of their manufacture, the repeatability of the manufacturing process used to form VCSELs, the reduced substrate area each VCSEL occupies, and because of the superior uniformity of VCSELs formed within the same substrate. Furthermore, vertical cavity surface emitting lasers typically require less power to drive their lasing action than edge emitting lasers.
One challenge associated with the use of VCSELs in conjunction with fiber optic module packages, is mounting the VCSEL in an optical subassembly (OSA) such that the VCSEL emits light along the fiber launch direction and therefore along the direction of the surface upon which the VCSEL is mounted. A VCSEL may be mounted on the surface such that it emits light orthogonal to the surface and such that mirrors are used to bend the light 90 degrees to try to focus the emitted light onto the end face of a fiber. Such bending of light diminishes the optical coupling efficiency and is undesirable. The optical subassembly including the VCSEL may also be mounted over the surface such that the VCSEL emits light substantially parallel to the surface. This typically involves mounting the VCSEL perpendicular to the surface, and the OSA on or over the surface. In this configuration, the additional components used to monitor the VCSEL and turn the electrical signal 90 degrees, are included in the OSA which is necessarily larger and therefore requires additional vertical space over the surface. There are space constraints in many applications that limit OSA designs, however, and such space constraints limit the vertical space available over a printed circuit board in which to mount an OSA.
It is therefore desirable to reduce the size of the overall module by as much as possible while maintaining the required electrical and optical functionality. In particular, it is desirable to reduce the height by which the OSA extends above the printed circuit board. A typical fiber optic module includes a high speed integrated circuit and OSA mounted on a common printed circuit board. The fiber optic module is desirably formed to a minimum size and optimized for best microwave performance.
There is also an inherent requirement in the art to optimize the thermal heat extraction away from the module sub-components in order to optimize performance. This can be done if the various components are oriented properly with respect to each other in the overall module housing. In particular, it is desirable to mount a heat generating component such as an integrated circuit, on the top surface of a board or other mounting surface since hot air rises and can more effectively be directed away from other components mounted on or below the board, or otherwise in close proximity.
Another motivation in this field is to increase integration levels and packaging density and to provide a high speed electrical signal path which is as short as possible. It is therefore desirable to have as many electrical input/output (I/O) terminals as possible on an IC (integrated circuit) chip package mounted on a printed circuit board, for example. A ball grid array (BGA) provides such a tightly packed array of multiple I/O terminals on an IC chip package. The individual conductive balls of the BGA are coupled to the actual IC chip by wire bonds and metal traces that are internal to the IC chip package.
Presently, one of the fundamental challenges in forming a microwave optimized, high-speed data path is to route a conductive pathway from a conductive ball that is centrally situated on the BGA of the IC chip package, to an OSA. When such an IC chip package is mounted on a PC board according to conventional arrangements, the centrally situated conductive ball must be routed to the periphery of the BGA and the package, in order to be coupled to a conductive trace formed on the surface of the PC board, that is coupled to the OSA and extends to the periphery of the IC package. This routing may be achieved by forming a conductive path extending through the BGA that is electrically isolated from the other balls in the BGA. One approach for addressing this challenge is to remove balls from the BGA and route a conductive path from the centrally situated conductive ball to the periphery of the BGA, and between the remaining balls of the BGA. This approach undesirably reduces the total I/O terminal count.
This arrangement also limits the amount of freedom in routing the conductive traces of the PC board from the IC package to the OSA because the conductive traces are formed along the top surface of the PC board, which is cluttered with the balls of the BGA that is included on the bottom surface of the IC package which is mounted on the top surface. In order to route the PC board traces along the cluttered top surface of the PC board, this approach favors the use of traces of minimal width. This, in turn, increases skin effect loss, resistance, and impedance. Such increases are detrimental to microwave performance.
It is therefore desirable to provide a fiber optic module which includes an optical subassembly including a VCSEL capable of emitting light substantially parallel to a printed circuit board surface, and such that the fiber optic module occupies a reduced amount of vertical space. It is also desirable to have as many electrical input/output terminals as possible on an IC package while electrically coupling an integrated circuit chip to an optical subassembly using conductive traces optimized for best microwave performance and which include the shortest possible electrical signal path.